Robert P. Swierenga. By the Sweat of our Brow: Economic Aspects of the Dutch Immigration to Michigan. swierenga.com. 13 March 1997, Lecture of Dr. Robert P. Swierenga, Research Professor, A.C. Van Raalte Institute for Historical Studies, Hope College, Holland Museum Sesquicentennial Lecture Series, Holland [2014-01-07]. （原始内容存档于2020-01-25）.请检查|date=中的日期值 (帮助)
Robert P. Swierenga. By the Sweat of our Brow: Economic Aspects of the Dutch Immigration to Michigan. swierenga.com. 13 March 1997, Lecture of Dr. Robert P. Swierenga, Research Professor, A.C. Van Raalte Institute for Historical Studies, Hope College, Holland Museum Sesquicentennial Lecture Series, Holland [2014-01-07]. （原始内容存档于2020-01-25）.请检查|date=中的日期值 (帮助)
